1. Field of the Invention
The present invention relates to a deviation compensation apparatus, and, in particular, to a deviation compensation apparatus compensating for at least one of amplitude deviation and phase deviation.
2. The Description of the Related Art
In recent years, a cellular mobile communication system in which a plurality of antenna elements (a multi-beam antenna, adaptive array antenna, etc.) are provided in a radio base station, and digital signal processing is performed on signals transmitted/received therethrough attracts attention.
When such a system employing a multi-beam antenna, adaptive array antenna system, or the like accompanied by digital signal processing is applied to a radio base station of a cellular mobile communication system, it is possible to equivalently sharpen a beam pattern so as to improve the gain, and, also, to reduce interference within the area due to the directivity thereof. As a result, the number of users which can be accommodated by one cell can be effectively increased.
However, in order to attain a beam forming system by signal processing in a digital domain, at a reception side, a low noise amplifier (LNA), a mixer for frequency conversion, etc. are needed in a process of converting a radio frequency signal (RF signal) received by each antenna into a baseband frequency signal. Moreover, also at a transmission side, nonlinear devices such as a frequency converter which carries out frequency conversion from the baseband frequency to the RF frequency, an RF high power amplifier (HPA), etc. are needed for each antenna branch. When a amplitude deviation and/or phase deviation occurs on these nonlinear elements independently for each antenna branch, efficient beam forming may not be performed, and degradation in the characteristic may occur.
Furthermore, for an up-link circuit (circuit from a mobile station to a radio base station), the phase on each antenna branch includes the phase between each antenna determined by directions from which user signals is incident in the communication area (cell or sector) to which the antenna is directed to and the arrangement of antennas of the base station. Accordingly, only the phase deviation should be compensated for while the phase difference information needed for array combination processing of each antenna reception signal should be maintained.
Furthermore, also in a down-link circuit (circuit from the radio base station to the mobile station), a weight is given for a signal provided to each antenna normally in the baseband for beam forming, and radiation should be made from the respective antennas while the weighting condition should be maintained. Therefore, only the phase deviation should be compensated for while the weighting condition should be maintained. Thus, compensation for amplitude and phase deviation is an extremely important matter for introducing such a system on multi-beam antenna or adaptive array antenna.
FIG. 1 shows an outline view of a configuration of a system which employs an adaptive array antenna. This figure shows a configuration of a receiving part. An LNA(s) (low noise amplifiers) 102a through 102d, frequency converters 103 and 105, amplifiers 104a through 104d, A/D converters 106a through 106d, multipliers 107a through 107d, and a combining part 108 are provided for a plurality of antennas 101a through 101d. The frequency converters 103 and 105 include an LO (local oscillators) and mixers.
From the LNA 102a, a signal received by the antenna 101a is output at low noise and high gain, and is converted into an intermediate frequency signal (IF signal) from the RF signal by the frequency converter 103. Then, after the IF signal amplified by the amplifier 104a is converted into a baseband signal by the frequency converter 105, it is converted into a digital signal by the A/D converter 106a, and is weighted by a weight W through the multiplier 107a. The same processing is performed also for the antennas 101b through 101d. The thus-weighted signals are combined by the combining part 108. The reception signal is expressed by a complex number having parameters of an amplitude ‘a’ and a phase θ. Similarly, a transmission signal is expressed by a complex number having parameters of an amplitude ‘a’ and a phase θ.
When the radio frequency signal is received from the incident direction φ shown in FIG. 1 to the antennas 101a through 101d, a phase difference based on difference in transmission path occurs in the reception signal. With respect to the antenna 101a, as shown in FIG. 1, for the antennas 101b through 101d, the transmission path differences A1 through A3, occurs, respectively, for example. For example, by setting the weights W such as to cancel these transmission-path differences by the multipliers 107a through 107d, and, the combining part 108 combines them, a beam pattern B1 as shown in FIG. 2 can be obtained as a beam pattern of this adaptive array antenna.
Generally speaking, the directivity of the adaptive array antenna can be set such as to have a strong directivity for a desired signal direction, and have nulls for non-specific interference directions. The beam pattern B1 obtained by such an adaptive array antenna is compared with a beam pattern B2 obtained by a single antenna receiving a signal, by using FIG. 2. Assuming that an incoming direction of a signal on a desired user is φ, an incoming direction of a signal on an interference user is η, and the signal levels on the desired user signal and interference user signal received by the respective beam patterns are P1, P2, and P3, P4. As a result, although there is no significant level difference La between P3 and P4 by the beam pattern B2, the level difference Lb between P1 and P2 is remarkably large by the beam pattern B1. Thereby, it is possible to improve the SIR.
Moreover, when the above-described system performs beam forming, as shown in FIG. 1, on reception, in order to convert the RF signals received by the respective antennas 101a through 101d into the baseband signals, the nonlinear devices such as the LNAs 102a through 102d and the mixers are needed. Moreover, although not shown in the figure, also on transmission, nonlinear elements which carry out frequency conversion of the baseband signals to the RF signals, such as frequency converter and HPA for RF signal, are needed for each antenna branch.
For this reason, generally speaking, a method of performing calibration between the respective antenna branches periodically (once a day, or the like) is performed according to the related art.
However, in case the amplitude and phase deviation occurs dynamically, beam forming is performed on indefinite phase conditions, and thus, the reliability of the system may not be maintained at a sufficiently high level. As a scheme of solving this problem, an article “A Calibration Method for DFB Receiving Array Antenna by Using Maximum-Ratio-Combining Weight”, Technical Report of IEICE (Institute of Electronics, Information and Communication Engineers), AP97-96, discloses to a method to be applied to an up-link array antenna system, and, also, another article “A Remote Calibration Method for DBF Transmitting Array Antenna by Using Synchronous Orthogonal Code”, Communication Society, IEICE, SB-1-17, 1998, discloses a method to be applied to a down-link array antenna system.
However, in up-link case, in order to extract the amplitude and the phase deviation occurring between respective branches, a certain signal should be transmitted from a known direction into a cell or a sector. In down-link case, known signals are needed on both transmission and reception ends, an orthogonal multi-beam should be used for transmission and, also, a deviation signal should be informed to the transmission source.
Moreover, in the related art, there are restrictions in layout of hardware, spaces, etc., and, in case deviation compensation processing is needed on every functional block, many signal wires which go back and forth between the processing blocks of each branch are needed, and, thus, as a result of arrangement of signal bus lines becoming complicated, it may become disadvantageous in respect of cost and/or reliability.
Moreover, in the related art, as deviation compensation processing is performed on each hardware functional block, a circuit configuration may become complicated in case the deviation compensation is processed for a different carrier frequency in order to reduce the number of signal lines disposed.
For example, in FIG. 3, amplitude and phase deviation compensation blocks 4a and 4b are blocks which perform amplitude and phase deviation compensation on the nonlinear elements 31 through 34, and, branching parts 1a and 1b, combining parts 2a and 2b, and circuits 5a and 5b are provided for respective transmission paths. Thus, the circuit configuration becomes complicated.